It has been recognized for many years that magnetic fields can affect the behavior of free radicals in heterogeneous media such as detergent solutions. For example, application of a static magnetic field (MF) during reduction of the benzophenone triplet state, 3[BP], by 1,4- cyclohexadiene in sodium dodecylsulfate (SDS) causes a dramatic increase in the yield of radicals that escape from the triplet pair. While geminate (ie born together) triplet radical pairs (G-pairs) are not frequently found in biological systems any random radical-radical encounter will give rise to a triplet radical pair (F-pair) with a 75% probability. Although the origins of F-pairs and G-pairs are different they are both affected by external magnetic fields in the same manner. Magnetic field effects on radical reactions are best observed in heterogeneous systems such as detergents where the reactants are confined in a small volume. In biological systems it seems likely that many of the cellular subcompartments (membranes, DNA, proteins, mitochondria, and other subcellular organelles) might function as "reaction vessels" and be influenced by an externally applied magnetic field. The aim of this proposal is to examine the effect of magnetic fields on free radical reactions that have some biological relevance. Free radical encounters in heterogeneous or organized media have durations in the microsecond time scale. In order to study the influence of a magnetic field on such reactions it has been necessary to construct a special detection system. We have assembled and tested a laser flash photolysis system based on an MY-33 Nd:YAG laser emitting at 1064, 532, 355 and 266nm with a 20ns resolution. This laser has been upgraded using an OPO tunable laser system s that we now have radiation at any wavelength between 400nm and 600nm. Currently we are employing optical detection to measure transients. We have also designed, built and tested an electrical conductivity cell which will be used to monitor transients that do not have suitable optical properties. Once the exposure system has been constructed we shall begin to examine the effect of magnetic fields on lipid peroxidation and the chemistry of NO.